1. Field of the Invention
The present invention relates to a dot printer.
2. Description of the Prior Art
In a dot printer, printing information for one page is expanded in dots on an image memory, and dot data is sequentially sent to a printing head to be printed. Usually, the data processing time for expanding one-page printing information in dots, and the time required for printing one page are nearly equal. To enhance the efficiency of the printer, using two image memories, a method of expanding the printing information of the next page in dots while simultaneously printing a first page has been proposed. Its shortcoming is, however, that two image memories are required. In particular, in the case of bills and labels affixed to the articles, the to distinguish articles document format or the various contents of different bills or labels are mostly the same, and are different only in part. When printing such printing information sequentially, almost the same printing patterns are repeatedly expanded on the dots in two images memories every time, which is very uneconomical.
In the continuous printing of multiple documents of the same type which differ only in part, a system that divides the printing information into a fixed portion and a variable portion has been proposed. In this system, the fixed portion and the variable portion are respectively expanded in the separate image memories, and the content of the image memory of the fixed portion is kept unchanged. By sequentially rewriting only the image memory of the variable portion, an OR operation between the contents of both image memories is effected and outputted to the printing head.
In such a system, for manually handling the printing content, it is convenient to treat the variable portion as a variable column in a document. Even in the variable column, however, the content of the column is not changed entirely in every document. For example, the date is variable, but the same date may be used over several pieces of a certain document. Even if the date varies in every document, only one digit of the date is changed in many cases. In such cases, it is a waste of time to rewrite the whole column every time. Besides, two image memories requiring a large memory capacity are needed.